docs: say which languages the widget ships

[anurag629]

What does this change?

The i18n page says the widget has 8 built-in locales. It has one: packages/widget/src/i18n.ts only registers en, and getWidgetLocale falls back to it for any other code. So data-locale="es" gives English buttons and placeholders without any warning. The 8 locale files live in @chatcops/core, and neither the widget nor the server uses them.

• widget/i18n.mdx now says English is the only built-in language, shows how to translate the widget with strings (that option already exists, nothing new here), lists only en in the table and links Add built-in translations to the widget #55 for built-in translations.
• Its "Server-Side i18n" section showed an i18n server option with Spanish strings. The server never reads that option. What it does is add "Respond in the user's language. Current locale: es" to the system prompt (packages/server/src/handler.ts line 115), so the section says that now. Error messages the visitor sees come from the widget's strings.
• widget/configuration.mdx: the comment on locale listed 8 codes.
• server/setup.mdx: the i18n line in the example now says the server doesn't read it yet.
• README.md: the feature line said 8 built-in locales.

Whether the unused i18n server option should go away or get wired up can be its own issue.

How I tested this

• pnpm build in website/ passes, and I read the rendered i18n page in the build output.
• The strings example from the page type-checks against WidgetConfig.

Checklist

• This PR targets dev
• Changeset added: not needed, only docs and the README change